This is an intensive longitudinal study of the development of 40 male and female babies between the ages of 12 and 24 months, with a follow-up assessment of their competencies at age 3 1/2. Its objectives are threefold: first, to document the developmental processes by which toddlers' experiences in encounters with many different facets of their environment become integrated into behavior reflecting their own representations of reality. Second, to determine to what extent individually characteristic patterns of response in transactions with the social and non-social world may become established in the course of different types of early experiences. Third, to trace the relationship between such types of developmental patterns as may be established in the second year and the domains in which the child subsequently evidences an expectation of competence in actualizing his own goals. Observations in the second year were conducted repeatedly and under a variety of conditions (at home, in small playgroups, in specially designed experimental situations, and while taking developmental tests). The bulk of these is recorded on videotape: through repeated re- viewing, methods for abstracting behaviorally detailed sequences of interactions with mother, other social objects, and various kinds of non-social stimuli are being established; preliminary reports of findings generated by these methods have been published or are in press. The behavior at age 3 1/2, observed mainly in the course of a special session of nursery school, is recorded in the form of Q-sorts and ratings along numerous behaviorally anchored scales. Continued analyses of interrelationships, changes, and continuities in different domains of functioning as shown by these forty babies are currently ongoing.